Hi! I'm the "longtime member"
referred to :)
Yes, my horse gets loose stool every winter, and it is much worse when he has to wear a blanket. He can't raise his tail quite as much as when he's naked.
The diarrhea went away with an antibiotic treatment called Metronidazole, but then it came back and we had to repeat the treatment. So far, he's back to normal, but it may return again, at which point I probably won't re-treat. I really don't like the idea of giving him all those antibiotics, but at the same time, he can't be absorbing a lot of nutrients when he has liquid poop. It's a balancing act. I will be satisfied with some improvement, and spring is on the way.
So tuck that in the back of your mind if your horse should continue to have this problem or if it gets worse. Before you resort to Metronidazole, however, I'd suggest probiotics. Sometimes that's enough. I had good luck with Bioequine's Digestive Supplement. Slippery elm bark has also helped my horse. The problem is that it would never completely go away, and got really bad this past winter. But he's an extreme case.
Another thing you can try with an aging horse that does better on grass than hay is steaming the hay. My horse loves steamed hay, and it makes the hay a lot more tender and full of water, much like grass. It requires some work, but you can make your own hay steamer out of a garbage can with lid and a humidifier. You just build a rack that sits over the humidifier and let the hay steam for about 45 minutes before you feed. It's not ideal for me because my horses live together in a herd so I'd have to steam everyone's hay. But if you can do it, it does help.